Storage tank for process liquids with a reduced amount of bubbles

ABSTRACT

In a storage tank for a process liquid, such as a plating liquid for a plating reactor, one or more barriers are implemented so as to define an inlet area and an outlet area, wherein a fluid flow path is substantially determined by the barrier to significantly suppress the migration of bubbles from the inlet area to the outlet area. In particular embodiments, the introduction of liquid into the storage tank is achieved by inlet lines terminating closely beneath the liquid surface so as to substantially avoid bubble generation and to remove moderately sized bubbles that are conveyed in the inlet lines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the field of fabrication of integrated circuits, and, more particularly, to manufacturing processes involving the application of process liquids, such as plating solutions, onto the surface of a substrate.

2. Description of the Related Art

In an integrated circuit, a huge number of circuit elements, such as transistors, capacitors, resistors and the like, are formed in or on an appropriate substrate, usually in a substantially planar configuration. In many stages of the manufacturing process, liquids, such as de-ionized water, specific chemicals, slurries and the like, have to be supplied to process tools so as to provide these process liquids to the substrate to accomplish a process under consideration. One important process stage during the formation of integrated circuits represents the field of forming metal-containing regions on a substrate by wet chemical processes, such as plating. Due to the large number of circuit elements and the required complex layout of the integrated circuits, generally, the electrical connection of the individual circuit elements may not be established within the same level on which the circuit elements are manufactured but requires one or more additional “wiring” layers, also referred to as metallization layers. These metallization layers generally include metal lines, providing for the inner-level electrical connection, and also include a plurality of inter-level connections, also referred to as vias, wherein the metal lines and vias may also be commonly referred to as interconnects. Furthermore, the connection of the integrated circuit or portions thereof to the periphery is usually established by a plurality of contact pads, which in sophisticated devices bear so-called solder bumps, enabling a direct connection with corresponding areas of a package substrate by means of reflowing the solder bumps.

Two frequently used techniques for depositing a metal on a substrate are electroplating and electroless plating. In the process of electroless plating metals onto a substrate surface that may have formed thereon circuit elements and a patterned dielectric or a photo-resist layer, a catalytic material may be formed prior to bringing the metal-containing solution into contact with the substrate surface. In the electroplating process, a current distribution layer is required to electrically connect the specified substrate regions that are intended to receive a metal with an external current source so that the metal-containing solution contacting the specified regions may be reduced and deposited as a metal. Typically, the plating process is conducted in a plating tool comprising a plating chamber in which the substrate is brought into contact with the plating solution. Although simple bath reactors may be used for this purpose, it turns out that, for sophisticated applications, a fountain-type reactor is the preferred tool for plating metal onto a substrate. Generally, a fountain-type plating tool comprises a process chamber and separated therefrom a storage tank containing the plating solution, which is conveyed via a conduit system to the process chamber. In the process chamber, the plating solution is applied to the substrate, which is placed with its receiving surface so as to face the electrolyte stream, wherein the excess solution is re-circulated to the storage tank. As in many other situations requiring a well-defined application of a process liquid onto a substrate surface, a non-uniform distribution of the process liquid across the substrate surface may remarkably affect the result of the process under consideration. In the case of the plating process, the resulting uniformity of the metal layer deposited and/or the quality thereof may significantly depend on how the plating solution is applied to the substrate.

For instance, bubbles in the plating solution have been identified as a substantial source of failures for integrated circuits, since the presence of a bubble in the solution stream may prevent or reduce the deposition of metal in bubble-containing areas of the substrate surface, thereby resulting in an erroneous chip or a premature failure of the integrated circuit. Furthermore, the presence of bubbles in the plating solution may lead to an increased oxidation of sensitive additives included in the plating solution, thereby changing the performance of the plating solution and thus affecting the entire plating process. Hence, the presence of bubbles in process liquids, especially in plating solutions, may represent a serious issue in terms of production yield and device reliability. In particular, the loss of per se functional devices in a late manufacturing stage, such as the formation of solder bumps, may drastically contribute to the production costs.

In view of the above-explained situation, a need exists for efficient means that enable preventing or at least significantly reducing the presence of bubbles in storage tanks and/or supply lines for process liquids.

SUMMARY OF THE INVENTION

Generally, the present invention is directed to a technique that significantly reduces the amount of bubbles existing or created in a storage tank for a process liquid, such as a plating solution required for the wet chemical deposition of metal layers or metal regions on a substrate. The present invention provides a storage tank having at least an inlet area and an outlet area and takes advantage of the fact that a liquid flow within the storage tank may be guided between the inlet area and the outlet area in such a manner that the probability for conveying bubbles from the inlet area to the outlet area is significantly reduced.

According to one illustrative embodiment of the present invention, a storage tank for a process liquid comprises an inlet area for receiving the process liquid and an outlet area that is connectable to a supply line. Furthermore, a barrier is provided that has an upper portion and a lower portion, wherein the lower portion faces the bottom of the storage tank. The barrier is disposed between the inlet area and the outlet area and defines, with its lower portion, an opening near the bottom of the storage tank to provide fluid communication between the inlet area and the outlet area.

According to still another illustrative embodiment of the present invention, a plating tool comprises a process chamber configured to receive and support a substrate. The plating tool further comprises a supply line and a discharge line, wherein the supply line is configured to convey a plating solution to the substrate and the discharge line is configured to remove excess plating solution from the process chamber. Additionally, the plating tool comprises a storage tank including an inlet area connected to the discharge line and an outlet area connected to the supply line. Moreover, a barrier is provided that has an upper portion and a lower portion, wherein the lower portion faces the bottom of the storage tank. The barrier is disposed between the inlet area and the outlet area and defines, with its lower portion, an opening near the bottom portion so as to provide fluid communication between the inlet area and the outlet area.

According to still a further illustrative embodiment of the present invention, a method of operating a storage tank for a process liquid comprises supplying the process liquid to an inlet area of the storage tank and generating a liquid flow to an outlet area via a communicating opening that is disposed in the vicinity of the bottom of the storage tank so as to substantially prevent bubbles from moving into the outlet area.

In other embodiments of the present invention, the storage tank comprises a second barrier having an upper portion and a lower portion, wherein the lower portion is connected to the bottom of the storage tank and the upper portion forms a second opening to provide a fluid flow path between the inlet area and the outlet area.

In other particular embodiments of the present invention, the inlet area comprises an inlet line having an end portion, wherein a height position of an opening of the end portion terminating into the inlet area is adjustable with respect to a liquid level in the inlet area.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:

FIG. 1 schematically shows a storage tank having an inlet area and an outlet area according to illustrative embodiments of the present invention; and

FIGS. 2 a-2 c schematically show a storage tank or portions thereof that is connected to a plating tool in accordance with further illustrative embodiments of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The present invention will now be described with reference to the attached figures. Although the various regions and structures of a semiconductor device are depicted in the drawings as having very precise, sharp configurations and profiles, those skilled in the art recognize that, in reality, these regions and structures are not as precise as indicated in the drawings. Additionally, the relative sizes of the various features and doped regions depicted in the drawings may be exaggerated or reduced as compared to the size of those features or regions on fabricated devices. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.

As previously explained, the presence of bubbles in process liquids, especially in plating solutions, may lead to significantly reduced production yields. Hence, considerable efforts are made so as to substantially prevent the generation of bubbles and/or to remove existing bubbles prior to providing the process liquid to the substrate. In some conventional process tools, such as in lithography tools, certain types of filters and active de-gasing elements are used which may consume considerable space and which may also be relatively expensive while at the same time requiring a moderately high amount of maintenance. Contrary to these conventional methods for removing bubbles from a storage tank, the present invention relies on the fact that a fluid flow path within a storage tank may be established that substantially avoids the formation of bubbles and also results in the removal of already existing bubbles in the process liquid. With reference to FIGS. 1 and 2 a-2 c, further illustrative embodiments of the present invention will now be described in more detail.

FIG. 1 schematically shows a storage tank 100 that is configured to contain a specified process liquid, such as a plating solution for a desired metal or metal-containing compound as may be required for forming metallization layers, solder bumps and the like. The storage tank 100 comprises an inlet area 110 and an outlet area 120. The outlet area 120 may be configured so as to be connectable to a supply line 121, which may in turn be connected to a pump 122. The inlet area 110 may comprise one or more inlet lines 111 a, . . . 111 c, each having respective end portions 112 with an opening 113 terminating in the inlet area 110. The outlet area 120 and the inlet area 110 are substantially separated by a barrier 130 having an upper portion 131 and a lower portion 132. The lower portion 132 includes or forms at least one opening 133 near a bottom 101 of the storage tank 100 that provides a fluid flow path, denoted by 150, between the inlet area 110 and the outlet area 120. The at least one opening 133 may be provided in the form of a gap between the bottom 101 of the storage tank 110 and the barrier 130, or the at least one opening 133 may be provided in the form of one or more distinct openings formed in the lower portion 132 in the vicinity of the bottom 101. In this respect, the terms “in the vicinity” and “near” are to be understood such that the opening 133 is located in the lower half and preferably in the lower quarter and more preferably in the lower 10 percent with respect to a height 102 of the storage tank 100. In one particular embodiment, the opening 133 forms the only fluid flow path between the inlet area 110 and the outlet area 120. The barrier 130 may be comprised of any appropriate material, such as metal with or without an appropriate coating, plastic materials, and the like, wherein, preferably, at least the surface of the barrier 130 is substantially inert with respect to the process liquid to be contained in the storage tank 100.

During operation of the storage tank 100, a specified process liquid 140 may be filled into the tank 100 so as to provide a desired liquid level in the tank 100. In one embodiment, the liquid level and/or the height position of the openings 113 of the inlet lines 111 a, . . . , 111 c is selected so that the openings 113 are immersed into the process liquid 140 with a specified distance to the liquid surface. For instance, if the process liquid 140 is supplied to the tank 100 via the inlet lines 111 a, . . . 111 c, a corresponding controllable pump (not shown) may be provided so as to control the amount of liquid supplied to the inlet area 110 in conformity with the liquid volume discharged by the supply line 121 to maintain a substantially constant liquid level within the tank 100. To this end, the storage tank 100 may contain a level detector (not shown) configured to provide a level dependent signal to the controllable pump source in the inlet lines 111 a, . . . 111 c. During the intake of the liquid 140 by the inlet lines 111 a, . . . 111 c, bubbles generated anywhere during the transport of the liquid 140 may be released through the openings 113, wherein at least moderately large bubbles may immediately contact the surface of the liquid 140 and thus may be effectively removed. Moreover, by separating the inlet area 110 from the outlet area 120 by means of the barrier 130, which provides a fluid flow path through the at least one opening 133 in the vicinity of the bottom 101, fluid flow of the liquid 140 is forced down to the opening 133, as indicated by the arrow 150, and thus moves against the direction of buoyancy of the bubbles, thereby significantly reducing the probability for conveying bubbles from the inlet area 110 to the outlet area 120 through the at least one opening 133. Hence, the process liquid 140 may be provided to any process tool with a significantly reduced amount of bubbles. Moreover, the simplicity of the configuration of the storage tank 100 results in low cost installation, i.e., mounting the barrier 130, and provides a substantially maintenance-free operation, except for the usual maintenance periods as are required for conventional storage tanks.

In other embodiments in which the liquid level of the process liquid 140 may change during operation, a corresponding height adjustment member may be provided for the inlet lines 111 a, . . . 111 c so as to enable a predefined distance of the openings 113 with respect to the liquid surface of the liquid 140. Corresponding examples for a height adjustment member will be described in detail with reference to FIG. 2 b. The provision of a height adjustment member enables a substantially constant effect in removing bubbles in the inlet lines 111 a, . . . 111 c, and/or substantially avoids the generation of new bubbles upon supplying the liquid 140 to the inlet area 110.

FIG. 2 a schematically depicts a plating tool 260 including process chambers 261 a, 261 b connected to respective supply lines 262 a, 262 b and respective discharge lines 211 a, 211 b. The process chambers 261 a, 261 b may be configured in the form of a fountain-type plating tool in which a substrate may be held in place so as to receive plating solution delivered by the supply lines 262 a, 262 b. The plating tool 260 may further comprise a manifold 263, which is connected with its output side to the supply lines 262 a, 262 b, and which is connected with its input side to a supply line 221 including a pump 222. Moreover, a filter 223 may be provided within the supply line 221. The plating tool 260 further comprises a storage tank 200, which in one embodiment (not shown) may have substantially the same configuration as previously described with reference to FIG. 1. In the embodiment shown, the storage tank 200 comprises an inlet area 210 and an outlet area 220 with a first barrier 230 disposed between the inlet area 210 and the outlet area 220. The first barrier comprises an upper portion 231 and a lower portion 232, wherein the lower portion 232 includes or defines at least one opening 233 for providing a fluid flow path from the inlet area 210 to the outlet area 220. Regarding the number and location of the at least one opening 233 and regarding the configuration of the first barrier 230, substantially the same criteria apply in this case as previously described with reference to the barrier 130 depicted in FIG. 1. The storage tank 200 further comprises a second barrier 270 disposed between the first barrier 230 and the outlet area 220, wherein the second barrier comprises a lower portion 272 and an upper portion 271 including or forming an opening 273 so as to provide a fluid flow path from the inlet area 210 to the outlet area 220 via an intermediate area 215.

It should be noted that the opening 273 is to be considered to include any physical opening formed in the upper portion 271 and also includes an opening that is formed by a reduced height of the second barrier 270 with respect to a desired liquid level 274, irrespective of whether a liquid 240 is actually contained in the tank 200 or not. Hence, an “effective” vertical size of the at least one opening 273 is defined by the configuration of the upper portion 271 and the liquid level 277. In one particular embodiment, the second barrier 270 is configured such that the opening 273, in the form of one or more openings in the above-defined sense, provides the only fluid flow path from the intermediate area 215, defined by the first barrier 230 and the second barrier 270, to the outlet area 220. Thereby, the vertical size of the opening 273 may be selected such that any bubbles in the liquid 240 flowing through the opening 273 become effectively in contact with the surface of the liquid 240, thereby removing bubbles from the liquid 240. In one particular embodiment, the opening 273 is, at least partially, formed by a gap between a corresponding end portion of the upper portion 271 and the desired liquid level 274, thereby providing an increased probability for any bubbles to contact the surface of the liquid 240 during liquid flow through the opening 273.

In other embodiments (not shown), two or more intermediate portions 215 may be provided in that sequentially a first barrier 230 having an opening 233 at the lower portion 232 and a second barrier 270 having an opening 273 in the upper portion 271 are arranged so as to define a substantially zigzagging flow path from the inlet area 210 to the outlet area 220. In this way, the capability for bubble removal may be increased compared to that of a single intermediate area 215 as shown in FIG. 2 a.

During the operation of the plating tool 260, the liquid 240 may be supplied with the desired liquid level 274 to the tank 200 via the discharge lines 211 a, 211 b or any other liquid supply lines (not shown). Thereafter, the pump 222 is activated to provide a stream of liquid 240 via the supply line 221, the manifold 263, and the supply lines 262 a, 262 b to the process chambers 261 a, 261 b. Excess liquid that is not consumed during the plating process in the process chambers 261 a, 261 b is discharged via the discharge lines 211 a, 211 b and is supplied to the inlet area 210 via the openings 213, which are adjusted in height so as to be maintained below the desired liquid level 274. As previously discussed with reference to FIG. 1, the discharge lines 211 a, 211 b may have respective end portions 212 having a bent area, which is, in one particular embodiment, provided in a substantially U-shaped configuration. Bubbles of moderate size created during the transport of the liquid 240 from the process chambers 216 a, 216 b to the inlet area are effectively removed due to the close proximity of the position of the openings 213 with respect to the liquid surface. Moreover, the generation of further bubbles during the discharge of the liquid 240 into the inlet area 210 is effectively suppressed. Since the fluid flow path from the inlet area 210 to the intermediate area 215 is substantially determined by the opening 233, the probability for conveying bubbles within the liquid stream is significantly reduced as the bubbles tend to rise to the surface. Thereafter, the liquid stream is forced to move upwards since the fluid flow path from the intermediate area 215 to the outlet area 220 is substantially determined by the opening 273, wherein remaining bubbles are effectively brought into contact with the liquid surface, thereby further removing bubbles from the liquid stream. Finally, the liquid 240 is sucked into the supply line 221 in the vicinity of the bottom of the tank 200, thereby further reducing the probability for conveying any remaining bubbles, since the bubbles will preferably remain at the surface of the liquid 240. Hence, the liquid 240 is supplied to the process chambers 261 a, 261 b with a significantly reduced amount of bubbles, thereby providing an enhanced quality of plated metal regions due to the lack of bubbles on the substrate surface. Moreover, the reduced amount of bubbles in the supply line 221, the manifold 263, and the supply lines 262 a, 262 b causes a reduction in any adverse reactions of additives included in the liquid 240 so that the stability of the plating process is enhanced. As previously pointed out, the efficiency in removing bubbles may even be increased by providing two or more intermediate portions 215 so that any bubbles moving with the liquid stream are forced to periodically move to the liquid surface and move against their own buoyancy to the bottom of the tank 200.

It should be appreciated that the shape of the first and second barriers 230 and 270 may be selected as is deemed appropriate for manufacturing the barriers 230, 270 and mounting the same within the storage tank 200. For instance, any support members may be formed, integrally or separately, on the barriers or may be attached thereto as is required for reliably fixing the barriers 230, 270. Moreover, the shape of the barriers may be selected in view of criteria such as reduced flow resistance, bubble trapping capability and the like. For instance, side walls of the barriers may be slanted with respect to the fluid flow path and edges defining the openings 233 and 273 may be rounded to reduce the flow resistance and to reduce the formation of a turbulent flow. In other examples, overhanging areas may be provided at the first and second barriers 230, 270, preferably in the vicinity of the respective openings 233, 273 so as to trap residual bubbles.

In some embodiments, screen-like elements (not shown) may be provided at the opening 233 and/or 273, wherein the size of individual screen elements is selected so as substantially prevent the passage of bubbles of a predefined size. In some embodiments, the liquid level 274 may be maintained substantially constant by continuously supplying additional liquid 240 to the tank 200, thereby compensating the amount of liquid consumed during the plating process in the process chambers 261 a, 261 b. To this end, an additional storage tank (not shown) may be provided and may be connected to the storage tank 200, preferably to the inlet area 210, wherein a controllable pump in combination with a level detector (not shown) may enable an appropriate control of supply of additional liquid 240. In other embodiments, corresponding height adjustment members may be provided for the end portions 212 and/or the first barrier 230 and/or the second barrier 270, as described with reference to FIGS. 2 b and 2 c so as to adapt the bubble removing effect to a varying liquid level 274.

FIG. 2 b schematically illustrates a portion of the inlet area 210 according to further illustrative embodiments of the present invention. As shown, the inlet area 210 comprises the discharge lines 211 a, 211 b and 211 c, wherein the end portions 212 of the lines 211 a, 211 b include a bent portion so as to form a substantially U-shaped configuration, or a siphon-like configuration. The discharge line 211 c may comprise an end portion 212 having a non-siphon-like configuration, such as a substantially linear configuration, in the vicinity of the output opening 213. The end portions 212 of the discharge lines 211 a, . . . 211 c further comprise flexible portions 214, for example in the form of a bellow-like portion or in the form of a flexible hose that allows itself to be readily bent in any required fashion. Moreover, the inlet area 210 further comprises a height adjustment member 216 including a floating body 217 and a support member 218. The support member 218 may be attached to the floating body 217 and to the end portion 212 so as to maintain a well-defined distance 202 between the opening 213 and the liquid level 274. In one particular embodiment, the support member 218 is configured such that the distance 202 may be readily adapted to any desired value prior to filling the storage tank 200 with the liquid 240. After having adjusted the distance 202, the floating body 217 insures that the desired distance 202 is substantially maintained during the operation of the storage tank 200, irrespective of a changing liquid level 274. Hence, the discharging of the liquid 240 into the inlet area 210 may be performed in accordance with the predefined distance 202, wherein the distance 202 may be selected equally or differently for different discharge lines 211 a, . . . 211 c. For instance, the liquid 240 may represent a composition of different materials, wherein one or more of these materials may be delivered by some of the discharge lines 211 a, . . . 211 c, wherein a difference in viscosity and/or pressure and/or amount may require a different distance 202 with respect to the liquid level 274 of the composite liquid 240. In other embodiments, when the discharge lines supply the excess plating solution from the process chambers 261 a, 261 b to the inlet area 210, different process recipes in the individual process chambers may require the discharge of process liquid under quite different circumstances. For example, a large amount of excess liquid may be discharged by one of the discharge lines compared to other discharge lines, or the amount and/or the size of bubbles created in one process chamber may significantly differ from other process chambers, thereby also requiring a different setting of the distance 202. In some embodiments, a guide member 219 may be provided, for instance in the form of correspondingly dimensioned baffles, so as to restrict the lateral movement of the floating body 217, thereby also stabilizing the lateral position of the end portions 212 and thus of the openings 213 of the respective discharge lines 211 a, . . . 211 c. Hence, a simple means for adjusting the height 202 is provided, wherein substantially no maintenance is required for the height adjustment member 216, as this member may be manufactured by durable materials, such as metal, coated metal, plastic materials and the like.

FIG. 2 c schematically shows a portion of the storage tank 200 according to further illustrative embodiments of the present invention. In the embodiments depicted in FIG. 2 c, the second barrier 270 comprises a flexible portion 275 at the lower portion 272 so as to allow a movement of the second barrier 270 at least in the vertical direction. For instance, the flexible portion 275 may be provided in the form of a bellow-like configuration, or the flexible portion 275 may be provided by any type of flexible material that may be readily bent so as to provide a certain movability, at least in the vertical direction. A height adjustment member 216 a is attached to the second barrier 270 by means of a corresponding support member 218 a. The height adjustment member 216 a comprises a floating body 217 a that is designed to provide a sufficient buoyancy with respect to the liquid 240 so as to be able to support the second barrier 270, thereby maintaining substantially constant the vertical position and thus the “effective” vertical size of the opening 273 with respect to the liquid level 274. By means of the height adjustment member 216 a, the vertical size 202 a of the opening 273 defined by the second barrier 270 and the liquid level 274 may be adjusted in any desired value and may be maintained substantially constant at this desired value during the operation of the storage tank 200 in a similar way as is described with reference to FIG. 2 b. If a plurality of second barriers 270 is provided, the individual dimensions 202 a may be adjusted in any desired fashion so as to increase the efficiency in reducing the amounts of bubbles provided to the plating tool 260.

In other embodiments, the first barrier 230 may comprise a height adjustment member 216 b including a support member 218 b coupled to the first barrier 230 and an adjustment screw 217 b that is configured to enable, in combination with the support member 218 b, a vertical motion of the first barrier 230 upon rotating the screw 217 b. In this way, the vertical size 202 b of the opening 233 may be adjusted in conformity with process requirements.

It should be noted that the embodiments described with reference to FIGS. 1 and 2 a-2 c may readily be combined in any appropriate manner so as to enhance the efficiency in suppressing bubbles in the process liquid supplied to a process tool, such as the plating tool 260.

As a result, the present invention provides highly efficient means for removing bubbles in a storage tank by controlling the fluid flow path within the storage tank by means of at least one barrier so as to sufficiently suppress the migration of bubbles from an inlet area to an outlet area of the storage tank. By means of at least one further barrier, the liquid stream is forced to flow at least partially in the vicinity of the liquid surface so as to significantly increase the probability for bubbles to come into contact with the surface, and thus to be removed from the liquid stream. In particular embodiments, the supply of liquid into the inlet area of the storage tank is accomplished such that the liquid is discharged into the inlet area closely beneath the liquid surface. Although a high efficiency in bubble removal is obtained, substantially no additional maintenance is required, contrary to conventional bubble removal means with active components such as active de-gasing elements. Moreover, the usage of expensive filter elements may be significantly reduced or rendered completely obsolete by the storage tank configuration according to the present invention.

The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below. 

1. A storage tank for a process liquid, comprising: an inlet area for receiving said process liquid; an outlet area connectable to a supply line; and a barrier having an upper portion and a lower portion, the lower portion facing a bottom of said storage tank, the barrier being disposed between said inlet area and said outlet area, the lower portion defining an opening near a bottom portion of said storage tank to provide fluid communication between said inlet area and said outlet area.
 2. The storage tank of claim 1, wherein said inlet area comprises an inlet line end portion terminating into said inlet area.
 3. The storage tank of claim 2, further comprising a height adjustment member connected to said end portion and being configured to adapt a height of an outlet opening of said end portion to a liquid level in said inlet area.
 4. The storage tank of claim 2, wherein said end portion includes a bent portion.
 5. The storage tank of claim 4, wherein said bent portion has a substantially U-shaped configuration.
 6. The storage tank of claim 1, further comprising a second barrier having an upper portion and a lower portion, the lower portion connected to a bottom of said storage tank, said upper portion of said second barrier defining a second opening to provide fluid flow path between said inlet area and said outlet area.
 7. The storage tank of claim 6, further comprising a second height adjustment member coupled to said second barrier, the second height adjustment member being configured to adapt a position of said second opening to a liquid level in said storage tank.
 8. The storage tank of claim 1 configured to contain a plating solution.
 9. A plating tool, comprising: a process chamber configured to receive and support a substrate; a supply line configured to convey a plating solution to said substrate; a discharge line configured to remove excess plating solution from said process chamber; and a storage tank including: an inlet area connected to said discharge line; an outlet area connected to said supply line; and a barrier having an upper portion and a lower portion, the lower portion facing a bottom of said storage tank, the barrier being disposed between said inlet area and said outlet area and defining with its lower portion an opening near the bottom to provide fluid communication between said inlet area and said outlet area.
 10. The plating tool of claim 9, wherein said discharge line comprises an end portion terminating into said inlet area.
 11. The plating tool of claim 10, further comprising a height adjustment member connected to said end portion and being configured to adapt a height of an outlet opening of said end portion to a liquid level in said inlet area.
 12. The plating tool of claim 10, wherein said end portion includes a bent portion.
 13. The plating tool of claim 12, wherein said bent portion has a substantially U-shaped configuration.
 14. The plating tool of claim 9, further comprising a second barrier having an upper portion and a lower portion, the lower portion connected to a bottom of said storage tank, the second barrier forming with said upper portion a second opening to provide fluid communication between said inlet area and said outlet area.
 15. The plating tool of claim 14, further comprising a second height adjustment member coupled to said second barrier, the second height adjustment member being configured to adapt a position of said second opening to a liquid level in said storage tank.
 16. A method of operating a storage tank for a process liquid, the method comprising: supplying said process liquid to an inlet area of said storage tank; and generating a liquid flow to an outlet area via a communicating opening that is disposed in the vicinity of a bottom of said storage tank to substantially prevent bubbles from moving into said outlet area.
 17. The method of claim 16, further comprising adjusting a height of an opening of an inlet line terminating in said inlet area to be located a predefined distance below a liquid surface of said process liquid.
 18. The method of claim 17, further comprising forcing said liquid flow between said inlet area and said outlet area to flow partially at the vicinity of said liquid surface to further remove bubbles in said liquid flow.
 19. The method of claim 18, further comprising adjusting a distance of said liquid flow flowing at least partially in the vicinity of said liquid surface to a predefined value.
 20. The method of claim 16, wherein said process liquid is supplied to a process chamber of a plating tool. 